Using ExoMars TGO/NOMAD observations to help constrain the GEM-Mars GCM gravity wave parameterisations

Gravity waves are a phenomenon that has been observed in several planetary atmospheres, including Earth, Venus, and Mars. They are formed when air is stably stratified and are triggered by wind flow over topography (orographic) or by weather events such as frontal systems, jet streams and convection (non-orographic). As a parcel of air is forced up by one of these mechanisms in stable air, buoyancy acts as a restoring force on the parcel causing oscillations. As the resulting wave propagates upward where the atmosphere is less dense, the amplitude grows and energy and momentum are transferred from the lower to the upper atmosphere. On Mars, gravity-wave induced density and temperature fluctuations have been observed by orbiting platforms (e.g. [1-5]) and during aerobraking [6-7] and from the surface [8]. Their effects are also seen in airglow imagery [9].

While the waves are relatively small, ranging in wavelength from tens to hundreds of kilometres, their impact through thermal and dynamical forcing on the climate can be quite large and therefore need to be accounted for in atmospheric models. Global models typically do not resolve these waves so their impact on the large-scale flow must be parameterised. These parameterisation schemes are poorly constrained (see [10] for an overview).

We present the first analysis of density and temperature perturbations in the ExoMars Trace Gas Orbiter (TGO) Nadir Occultation for MArs Discovery (NOMAD) Solar Occultation (SO) observations [11] to help constrain the GEM-Mars Global Climate Model (GCM) [12, 13].

The GEM-Mars GCM uses two parameterisations for orographic [14] and non-orographic gravity waves [15-17], originating from the terrestrial version of the model [18-20]. By comparing temperatures, mapping the perturbations and analysing the derived potential energy and gravity wave drag from the observations, we can then adjust the schemes’ tuning parameters to better match the NOMAD temperatures. For example, in the non-orographic scheme, the lower bound vertical wavenumber, which limits the maximum vertical wavelength of the spectrum allowed, can be adjusted.

We show that by adjusting the parameters in the schemes, we can better reproduce the temperatures in the 70-100 km altitude range, especially in the midlatitude to polar regions.

References :

1 England, S. L. et al., 2017

2 Vals, M. et al., 2019

3 Heavens, N. G. et al., 2020

4 Starichenko, E. D. et al., 2021

5 Starichenko, E. D. et al., 2024

6 Creasey, J. E. et al., 2006

7 Fritts, D. C. et al., 2006

8 Guzewich, S. D. et al., 2021

9 Altieri, F. et al., 2012

10 Medvedev, A. S. and Yiğit, E., 2019

11 Vandaele A. C. et al., 2018

12 Neary, L. and Daerden, F. , 2018

13 Daerden, F. et al., 2019

14 McFarlane, N. A., 1987

15 Hines, C. O., 1997a

16 Hines, C. O., 1997b

17 Charron, M. et al., 2002

18 Côté, J. et al., 1998

19 Côté, J. et al., 1998

20 Yeh, K.-S. et al., 2002